Stage types

I added a field called `Type` to `PipelineStageStat`.  The built in values are as follows.  These are automatically configured by falcore, but are modifiable by developers.  The main idea here is to be able to separate out different types of stages.

An example of where I'd use this is to identify the last upstream stage run and include that in our metrics reporting.

```go
type PipelineStageType string

const (
	PipelineStageTypeOther      PipelineStageType = "OT"
	PipelineStageTypeUpstream   PipelineStageType = "UP"
	PipelineStageTypeDownstream PipelineStageType = "DN"
	PipelineStageTypeRouter     PipelineStageType = "RT"
	PipelineStageTypeOverhead   PipelineStageType = "OH"
)
```